In some alternating current (AC) electrical systems it may be beneficial to provide for stabilization of the output voltage. Known devices and methods for stabilizing AC output voltage may typically require conversion to direct current (DC) and then back to AC, which may result in inefficiency and high production costs. Other devices may use variacs, electromechanical devices, and/or components with ferro-resonance characteristics. Yet other systems may use or include an uninterruptible power supply (UPS), however, the UPS may be too large and/or expensive for many applications that may benefit from stabilized or regulated AC output voltage. Accordingly, there is a need for an efficient and inexpensive method and device for AC voltage stabilization.
FIG. 1 illustrates a step-down synchronous buck converter arrangement 100, also referred to as an electronic transformer or line conditioner. In operation, AC source 101 may provide an input voltage signal. The voltage at reference node 102, which may be a signal voltage-divided from the input voltage signal, may be fed to pulse width modulator (PWM) module 108. The operation may use a synchronous PWM controller 108 to provide the control signals for switches 103 and 104, via signals at outputs Qa and Qb, where Qa and Qb are substantially complimentary. Switches 103 and 104 may be controlled by the PWM or synchronous converter 108 to chop the input voltage. Inductor 105 and capacitor 106 comprise a basic output filter that may filter the voltage and provide the load 107 with stable AC voltage. Accordingly, based on the pulse width modulation, which drives the switching scheme, load 107 may be provided with an output voltage amplitude that is less than the input voltage.
FIGS. 2(a), 2(b), 2(c) and 2(d) illustrate examples of implementations of bidirectional switches that may be used in connection with an AC/AC converter, including using field effect transistors (FET) and bipolar junction transistors (BJT), as well as diodes.
In the example provided in FIG. 1, the output voltage amplitude at load 107 may be:Vout=Vin×DC,  (1)where DC represents the duty cycle of signal Qa, e.g., the time Qa is conducting as a fraction of the total period of the signal.
In the configuration of FIG. 1, due to the output filter constructed using inductor 105 and capacitor 106, the output voltage will be delayed relative to the input voltage, and therefore, the output voltage may be out of phase with the input voltage signal, thereby producing harmonic distortion and/or phase distortion. Since the control loop of the circuit configuration may be referenced to the input voltage signal, the circuit will try to obtain an output voltage signal in phase with the input voltage signal, which, as described in further detail below, may cause distortion at or near the zero crossing of the input voltage, as seen at FIG. 3, below.
FIG. 3 is a graph 300 plotting input voltage 320 and output voltage 330 along time axis 310 for a circuit such as the one depicted in FIG. 2. Output voltage amplitude may be less than that of the input voltage amplitude by a factor equal to the duty cycle, e.g, 50% for a duty cycle of 50%. The output voltage may be delayed or out of phase with respect to the input voltage by t=τ, where τ may be determined by the characteristics of the output filter, for example, the inductance and capacitance values of the output filter.
When using a PWM regulator for line conditioning applications, the output voltage may be phase-locked to the input voltage, for example, in order to achieve smooth transitions in the case of bypass conditioning and small phase margins between the three phase circuits. In some cases where output voltage must be in phase to input voltage, a closed-loop control is appropriate. Closing the control loop for zero delay output voltage with respect to input voltage, however, may result in the duty cycle demand as shown in FIG. 4.
FIG. 4 depicts a graph 400 of the duty cycle 420 varying along time axis 410 that would be required in order to provide for an output voltage having no phase delay with respect to input voltage, where Equation (1) is rewritten as DC=Vout/Vin. In the case of closed-loop control, a portion of the output voltage is sensed and compared to the input voltage to produce an error voltage for the control loop. As seen with respect to the graph of FIG. 4, the required duty cycle may approach positive infinity 430 just before the zero crossing of the input voltage and re-appear at negative infinity 440 just after the zero crossing of the input voltage signal. Such demands may produce clipping at the high and low boundaries of the feasible duty cycle, e.g., 100% and 0%. In addition, in real-world applications, it is difficult if not impractical for the control loop to handle an instantaneous change from positively infinite required duty cycle to a negatively infinite required duty cycle, or, for example, 100% duty cycle to 0% duty cycle. Accordingly, the output voltage may contain errors and total harmonic distortion (THD) may result.